what it does is if for example the window width is 855, then 855%2 equals 1, so it will set the window width to 855-1 = 854.
what this does effectively is it lets you make the window size always be an even number without using any conditions.
It's important for the window size to only ever be even numbers in pixelart games because origin points of sprites is always effectively between two pixels, so if you try to center it, it needs a pixel on the left, and a pixel of the right.
if your screen is an odd amount of pixels then the sprite does not know in which pixel on the screen to render so it keeps shifting between the left and the right one and you get artifacts.
Also be careful because if you import a sprite in construct 3 with an odd width or height, it'S origin point will be set on a pixel instead of between two pixels on that odd axis to make it centered, and that one sprite will get artifacts because it does not know where to render.